1. Field
This disclosure relates to output drivers, and more particularly to output drivers with selectable output impedance.
2. Related Art
As speeds continue to increase in integrated circuits, the issues relating to termination of the inputs and outputs are becoming more significant. For example, an output of an output driver of an integrated circuit can, at very high speeds, ring if the output impedance does not properly correspond to the input impedance of the circuit that it drives. The necessary control of the output impedance is difficult to achieve with an integrated circuit because of the variations in circuit performance caused by variations in processing. One technique has been for the output driver to have a variable output impedance that is selected to be a particular value. With the value being known with sufficient accuracy, the output can be terminated with an impedance that avoids the ringing. There are a variety of known output drivers with variable output impedance that allow for selection of a desired output impedance. One of the continuing issues is in providing a technique that selects the output impedance. In a typical case, the selection process and the ultimate operation of the output driver require an external resistor. The resistor is external because it needs to be precise in its value and not vary with temperature. Due to process variations, resistors on integrated circuits are not generally available with sufficient precision as to its value. The use of the precision resistor, however, does not completely alleviate issues relating to process variation. The output impedance, even though derived from a precision resistor, is still subject to process variations causing differences in transistor characteristics and corresponding differences in circuit performance. Circuit techniques can be used to reduce the differences in circuit performance but they are difficult to completely remove. Thus, the provided output impedance may still vary from what is desired.
Thus there is a need for an output driver with a desired output impedance that overcomes or improves upon one or more of the issues raised above.